Energy balance represents the complex interplay between the fuel we consume and the energy we exert, which makes this balance integral to the process of losing weight.

That reality inspired esteemed researchers representing the American Society for Nutrition and the International Life Sciences Institute to create a consensus statement highlighting state-of-the-art research on energy balance (Hall et al. 2012). This column will summarize their consensus and do the following:

Discuss the interactions between the components of energy balance and how they are regulated.

Review the accuracy of popular beliefs on energy balance.

Point out the limitations of current research findings on energy balance.

How Much Energy Do We Absorb From the Foods We Eat?

Our diets derive energy from fat, protein, carbohydrate and to some degree alcohol. As little as ~2%–10% of total food intake becomes waste to be excreted. The rest is biologically broken down (or oxidized) for the body’s energy needs, including growth, cell maintenance, physical activity, pregnancy and lactation, and other cellular life processes (Hall et al. 2012). The caloric yields for fat (9 kilocalories/gram), carbohydrate (4 kcal/g) and protein (4 kcal/g) refer primarily to the amounts of fuel these sources provide to the body’s cells for physiological processes. Some foods, depending on their type of fiber content, are not digestible and thus are not absorbed and used. Also, some types of cooking can inhibit absorption of certain foods, thus limiting how many calories get into the body.

What Are REE, TEF and AEE?

Energy balance components that are commonly of interest in weight management include resting energy expenditure, the thermic effect of food and activity energy expenditure.

Energy at Rest Resting

Resting energy expenditure, or REE, is the nonexercise energy that the body uses to survive. REE represents two-thirds of the body’s energy expenditure. It varies widely among people, primarily because of body size (the greater the body mass area, the more REE the body needs) and body composition (fat versus muscle). However, some of the variability in REE (~250 kcal/day) has not yet been fully explained (Hall et al. 2012).

Interestingly, the brain, heart, kidneys and liver represent relatively small amounts of body weight but demand significant energy for life, so they contribute meaningfully to the body’s total REE (Hall et al. 2012).

Energy for Digestion

The thermic effect of food, or TEF, signifies the energy expenditure associated with processing and digesting the foods we eat. Protein elicits the highest TEF, followed by carbohydrate and then fat.

Energy for Activities

Activity energy expenditure, or AEE, is the fuel the body uses for structured exercise and nonexercise movement, such as shopping, moving, doing daily chores and fidgeting. AEE may be the energy balance component that varies most widely among people, as many of us move a lot during our waking hours and get a considerable amount of daily exercise, while others live primarily sedentary lifestyles.

How Much Energy Does the Human Body Store?

On average, the human body stores 130,000 kcal of fat, primarily in the form of triglycerides. According to Hall et al. (2012), a lean adult may have ~35 billion adipocytes, or fat cells, while an extremely obese individual may have 140 billion adipocytes. In addition, the fat cells in obese people store two to three times more triglycerides.

Carbohydrate is stored in the form of glycogen, which is bound to water in the liver and muscle. Changes in carbohydrate storage often result in meaningful shifts in fluid storage; the more carbohydrate is eaten and stored, the more fluid the body retains.

Fat, however, does not need to bind with water to be stored in the body, and protein needs very little water for storage. Therefore, a person eating a higher percentage of carbohydrate, but not necessarily more kilocalories, will retain more water, thus increasing his or her total body weight, owing to extra water retention.

Regulation of Energy Expenditure: Is There a Set Point?

A positive energy balance happens when we take in more kilocalories than we expend. Over time, a positive energy balance will result in overweight and/or obesity. The interactions of REE, TEF and AEE may be affected if a person is in a positive (weight gain) or negative (weight loss) kilocalorie consumption mode.

Scientists feel the body has either a “set point” or a “settling point” to regulate body weight (Hall et al. 2012). The set point theory suggests that our bodies have a highly organized feedback-control system that tries to optimally regulate food intake and energy output and maintain our weight near a certain level. Most scientists believe that leptin (a hormone secreted by fat cells) feedback to the hypothalamus in the brain has the predominant role in regulating body weight. The settling point hypothesis is very analogous to the set point theory, only it suggests that the body regulates energy intake and output with a less orderly feedback system. Indeed, the body’s mechanism for regulating energy storage is still a matter of intense investigation among scientists and may represent a combination of both theories.

Hall and colleagues note that owing to daily changes in energy intake (meals we eat) and energy output (activity and exercise), humans are regularly in a state of energy imbalance—going back and forth from positive to negative.

How Do We Know When to Eat and When to Stop Eating? Satiety,

the feeling of fullness from a meal, results from sensory receptors in the body noting an enlargement of the gastrointestinal tract and sending a message to the brain that the stomach is full. Specialized intestinal hormones also send “feeling full” messages to the brain.

Between meals, the stomach will start producing and secreting the hormone ghrelin. Ghrelin is referred to as an orexigenic hormone, meaning it stimulates appetite. Rising secretions of grehlin create signals that tell the brain it is time to eat. As Hall et al. (2012) note, many people are highly affected by cognitive stimuli (like advertising) that encourage food intake, regardless of whether the body needs food or energy.

Where Does Exercise Fit in the Energy Balance Equation?

A major step in creating a negative energy balance to lose weight is adding exercise to a person’s daily life. However, Hall et al. (2012) note that with some people, adding extra physical activity at certain times of day leads to less activity during other times.

This scientific knowledge is extremely useful to personal trainers designing exercise programs for weight loss. Physical activity program designs need to include ways to help clients move more throughout the day in addition to doing structured exercise. Hall et al. observe that some studies show people may expend up to 500 calories daily through spontaneous movement.

What About the Future of Energy Balance Research?

Energy balance is a complex integration of several bodily mechanisms (see Figure 1). Up to now, long-sustained research on this phenomenon has not been completed. Eventually, when scientists develop new methodologies to study long-term energy balance, many of the unanswered questions about successful weight loss may be answered.

Figure 1. Complex interactions affecting energy balance

Questions and Answers on Energy Balance

Q. Does a “slowed” metabolism explain the weight management plateau that is often observed after 6–8 months of weight management?

A. According to Hall et al., the research indicates this plateau is more attributable to a failure to comply appropriately with the diet.

Q. Are some people obese because they have a slow metabolism?

A. Hall et al. advise that this is typically not true. If energy intake does not match energy output, the resulting disproportion will produce weight gain (if food intake is greater) or weight loss (if physical activity is greater).

Q. How much does exercise “afterburn” add to caloric expenditure?

A. Hall et al. say that postexercise energy expenditure (above resting values), called excess postexercise oxygen consumption, amounts to ~6%–15% of the energy expended during the exercise session. So if energy expenditure during an exercise bout was 300 kilocalories, EPOC would range from 18 to 45 additional kilocalories. EPOC is essentially the energy used to restore the body to its preexercise physiology.

Scientists Debunk “3,500 Calories Burns a Pound of Fat”

Q. Should exercise professionals still rely on the adage that burning 3,500 kcal results in the loss of a pound of body fat?

A. No. Hall and colleagues say this adage is not valid, because no time period has ever been associated with this weight loss strategy. The “3,500 rule” implies a linear association between calorie burning and weight loss. This linear association does not exist, because how bodies adjust and compensate over time to reduced caloric intake and increased energy output varies widely.

Furthermore, some people will lose a meaningful amount of muscle mass during caloric restriction–only diets, and this will also have a direct effect on metabolism. Hall et al. argue that consistent, small-to-moderate decreases in energy intake, combined with consistent increases in energy output (via structured exercise and spontaneous movement during the day), over time yield realistic improvements in weight management.

About the Author

Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico in Albuquerque, where he recently won the Outstanding Teacher of the Year award. Len w...